1. Field of the Invention
The present invention relates to an apparatus for scattering fibrous material, e.g., chips, said apparatus serving to spread chips using either a throw or air jet spreading method to form a mat of chips onto a moving band conveyor or the like, whereby the scattering process avails a gas flow, such as air flow, for instance; the apparatus having a scattering chamber mounted above the band conveyor or the like.
2. Description of Background Art
Scattering chips by rollers is basically known from patent publication FI-90746, wherein an apparatus is disclosed for spreading fibers or chips together with a binder into a mat onto a forming band conveyor, said apparatus comprising one array of rollers comprising at least three mutually parallel rollers, whereby the interroller spacings are adapted adjustable. An air flow is adapted to pass between the roller array and the band conveyor by means of air suction or using a combination air blow and suction.
From patent publication FI-20040698 is known a method and apparatus using the same for scattering chips with a binder into a mat of particles onto a band conveyor, in which apparatus the chips are scattered by passing them through one or more roller arrays and simultaneously a gas flow is employed wherein the gas is, e.g., air and the gas flow is passed into at least two substantially chamber-like spaces situated above and below said at least one roller array and in which at least one chamber-like space has the gas flow directed opposite to the gas flow direction in the other chamber-like spaces.
As compared to air-jet spreading, roller scattering is characterized by a good accuracy of scattering (i.e., small variations of basis weight of the mat) inasmuch as the classification of the chips is chiefly performed mechanically by means of the roller array rather than with the help of an air flow as in air-jet spreading. In air-jet spreading, the control of air flow patterns is particularly problematic. The air flow readily tends to become excessively turbulent thus degrading the accuracy of scattering (hence, the quality of scattering), because strong turbulence deviates the chip particles in an uncontrollable fashion, particularly in the cross direction of the particle mat.
In prior-art roller scattering constructions, it has been possible to keep the free gravity fall of chips in the chamber enclosing the roller array and the band conveyor advantageously very small (typically 300 mm, for instance). Air suction has been necessary only under the chamber. The velocity of the suction air flow has been maintained reasonably low (typically below 1 m/s). Due to the shallow structure of the chamber and the relatively small volumetric rate of air flow adapted to pass below the roller array has remained at an advantageously low level. As a result, the turbulence occurring in the air flows and hence tending to degrade the accuracy of scattering has been kept sufficiently low, whereby the accuracy of scattering has been good.
Today, the surface quality of particle board must fulfill increasingly higher specifications in certain surface treatment applications (particularly those aiming to cut costs). One such application is so-called “direct printing” wherein onto the surface of the board that is pretreated with a thin primer layer is printed, e.g., a wood grain imitation pattern directly using a multicolor printing method. In order to reduce product costs, the coat layers or paint layers applied onto particle boards are today preferably made thinner than previously. Board types suited for such coat application must have an extremely dense surface texture and be comprised of particles so fine that all the chips can typically pass through screen openings of, e.g., 1 mm square and, furthermore, of this chips typically 70%, for instance, can pass screen openings of 0.5 mm square. A particularly critical requirement is that such particles to spread must be sufficiently thin, e.g., max. 0.3 mm thick. The demand for chip thinness is in turn linked thereto that a particle possibly detached from the board surface during sanding or, e.g., edge trimming, may not leave an excessively deep dent that later could become visible as, e.g., a surface defect after the application of a thin coat or as a disturbing ragged edge after trimming a coated board by a saw. A thin chip that advantageously has a leaf- or fiber-like shape also reduces the porosity of the board surface thus, e.g., cutting down paint consumption during coating and improving the strength of the glue-to-chip bond, whereby the separation of chips is diminished. Conversely, a thick chip having, e.g., a cubical shape is inferior in this respect.
The irregular turbulence of air flows that degrades the accuracy of scattering is accentuated in conventional spreading by air jets which requires relatively high air plenums (typically higher than 2 m) together with air flow velocities typically faster than those employed in roller array scattering, and further, particularly, the use of active blowing at the feed end of chips in order to attain sufficient classification. Generally, the air-jet plenums must also be complemented with screens serving to damp turbulence of air flow. Such screens are clumsy to use, cause extra costs and are readily plugged as they must be placed in a dusty space. Due to dust generation by the active air jet blowing, the maintenance need of an air-jet spreading system is extensive as compared with roller array scattering. The production line must be stopped frequently for cleaning the air-jet nozzles and screens in order to restore the scattering accuracy to a reasonably good level.
To fulfill the earlier discussed requirement of using thin chips, the drop height of chips in roller array scattering must be increased case-by-case so much that a sufficiently large fraction of thin chips can be classified apart and, at the right moment, caused to drop onto a desired area of the particle mat. Increasing the scattering chamber height, however, promotes the turbulence of the air flow being sucked/blown into the chamber, whereby the accuracy of scattering is degraded.
In FI Pat. Appl. 20060437 a roller array scattering apparatus is disclosed, wherein an attempt is made to improve the accuracy of scattering with the help of an element which is placed in front of the air inlet opening located at the exit end of the roller array in the travel direction of the wood chips in order to homogenize the air inlet flow pattern. The flow-homogenizing element comprises, e.g., a drilled plate, screen, honeycomb structure or a tangential blower or the like capable of producing a substantially laminar air flow pattern, or a combination of any of these. Such an arrangement has been found to reduce lateral turbulence, particularly in the vicinity of the flow-homogenizing element. However, its effect cannot be extended over the entire volume of the scattering chamber, but instead, the transverse turbulence more remotely from the homogenizing element increases in a disturbing fashion.
For use in air-jet spreading systems are known air blow diffusers having in the close vicinity of the diffuser outlet mounted short, parallel guide vanes with a height substantially equal to that of the diffuser and aligned vertically in the travel direction of the forming line, whereby the vanes orient the air flow being blown into the scattering chamber. The homogenizing effect of such guide vanes, however, does not extend to the actual working space of the scattering chamber where a major portion of the particulate material being spread falls onto the conveyor thus forming a mat.
In summary it can be said that the scattering chamber height is preferably maximized to improve the classification of particles. However, a higher scattering chamber invokes detrimental lateral turbulence, whereby the cross-machine profile of the particle mat cannot be made sufficiently homogeneous due to the uncontrolled landing of the particles onto the conveyor across its cross-machine width. Turbulence is primarily invoked by the impingement of the air flow on the material flow. Obviously, turbulence increases when the scattering chamber is made higher and the air flow velocity is increased.